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Threats from Outer Space? Asteroids, Comets and Debris by Harold Geller Department of Physics and Astronomy George Mason University

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What Im Going to Talk About zHistorical views zDefinitions and distinctions among comets et al. zMisconceptions of comets, asteroids, etc. zWhat these things look like zWhere these things come from zThe Damage Hazard and The Torino scale zThe saga of SL-9 and other close calls zReferences and review

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Historical Views zIn 1902, Richard Proctor stated (regarding meteors and comets): There are few more interesting chapters in the history of astronomy than that which deals with the gradual introduction of meteors into an important position in the economy of the solar system. Regarded for a long time as simply atmospheric phenomena (though many ancient philosophers held another opinion), it has only been after a long and persistent series of researches that they have come at length to be regarded in their true light. zHe went on to say (specifically about comets): We know that the dimensions of these objects are in many cases enormous. We know, further, that there must be many thousands of comets remaining undiscovered for each that our astronomers have detected. And, lastly, we are led to recognize the observed association between certain meteor- systems and certain comets as indicative of a general law by which, in some way as yet unexplained, comets and meteors are associated together.

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Definition of Comet zComet [according to Funk and Wagnalls Standard Desk Dictionary] - A celestial body moving in an orbit about the sun and consisting of a nucleus of more or less condensed material, accompanied by a tenuous coma pointing away from the sun.

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Definition of Asteroid zAsteroid [according to Funk and Wagnalls Standard Desk Dictionary] - Any of several hundred small planets between Mars and Jupiter; also called planetoid.

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Definition of Meteor zMeteor [according to Funk and Wagnalls Standard Desk Dictionary] - A meteoroid that on entering the earths atmosphere at great speed is heated to luminosity and is visible as a streak of light; also called a shooting star.

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Definition of Meteoroid zMeteoroid [according to Funk and Wagnalls Standard Desk Dictionary] - One of the pieces of matter moving through outer space, that upon entering the earths atmosphere form meteors.

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Definition of Meteorite zMeteorite [according to Funk and Wagnalls Standard Desk Dictionary] - A portion of a meteor that has not been completely destroyed by combustion and has fallen to earth.

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Misconceptions about things that go boom z[Adapted from David Levys book Comets: Creators and Destroyers] z1 - It cant happen to us. Things wont change after a major impact. z2 - Any object that hits the Earth could cause global devastation. z3 - To prevent an impact, we have to destroy the comet or asteroid. [Adapted from David Levys book Comets: Creators and Destroyers]

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Misconceptions about things that go boom z4 - The chance that a comet or asteroid that could damage the Earths ecosystem will land in our lifetime is virtually zero. z5 - Earth is just as much at risk now as it was in the past. z6 - Impacts are bad for life. z7 - Every mass extinction was caused by an impact. [Adapted from David Levys book Comets: Creators and Destroyers]

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Misconceptions about things that go boom z8 - An object the size of the dinosaur comet cannot threaten the Earth today. z9 - Life began on comets. z10 - Impacts are science fiction; they dont really happen in the solar system. [Adapted from David Levys book Comets: Creators and Destroyers]

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Where did they come from? zKuiper Belt yJust beyond reaches of solar system, once thought to be location of origin of comets. yLikely source of Jupiter family short-period comets. zOort Cloud yLikely region of most comets, located far away from solar system (25,000 - 100,000 AU). yThese comets were likely formed closer in, but their orbits were influenced by the Jovian planets. yPossible location of a Brown Dwarf (Matese, 1999).

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A Quick Review of Asteroids zCategorized as family of objects ybetween the orbits of Mars and Jupiter ycan be in other inner solar system orbits zA part of our solar system zCan go boom if you bump into them zOf interest in study of primordial stuff yinner solar system stuff, rocky material yhave been found with satellites of their own

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A Quick Review of Comets zObserved by humans for generations zOriginally considered as signs of bad fate zThe source of common meteor showers zA part of the solar system zKuiper Belt and Oort Cloud parking lots zCan cause a boom in the night (or day) zOf interest for primordial matter studies